In the tobacco and food processing industries the tobacco or food products are customarily conveyed on conveyor belts from one processing step to the next and then to their eventual packaging. These belts have at least one bottom traction layer, which normally is a fabric, and which forms the backside layer these belts, i.e. their side opposite to the conveying side and running over the pulleys. In order to avoid penetration of moisture into these conveyor belts, which would favor the formation of mould, the bottom fabric is customarily sealed from the environment by applying an impregnation of a crosslinked acrylate polymer to it. Since the impregnation is then the material coming in contact with the pulleys, it is prone to form abraded acrylate-containing dust particles which may contaminate the tobacco or food products conveyed on the belt. In view of the ever more stringent consumer product safety regulations such acrylate contamination of the conveyed tobacco and food products becomes a growing concern.
In British patent specification No. 1,013,081 a conveyor belt for food applications comprising a carcass formed of a number of woven cotton fabrics bonded together by rubber or other elastomeric material is disclosed. The conveying surface, or both surfaces, of the belt were covered by a layer of pure polypropylene. The polypropylene was applied as a polypropylene sheet with a geometric thickness of 0.005 inch (=0.127 mm), by bonding by fusion and pressure to the fabric(s). The fabric(s) had been pretreated by exposure to an atmosphere of 63% to 67% relative humidity for 24 hours. The propylene covering layer(s) are said to have good adhesion to the fabric(s). The polypropylene coating also appears to withstand tensile forces and yet to have adequate flexibility.
Aqueous polyolefin dispersions have been known from e.g. JP 05/105791 A and US 2005/100754 A. These dispersions have been made by melt-kneading the polyolefin in an extruder together with a dispersing agent and water. The dispersing agent may be a monomeric surfactant or a polymer having ionizable groups, in particular anionically ionizable groups such as carboxylic acid groups. These dispersions have been used so far for coating paper or cardboard or for impregnating the backside of carpets, or have been converted to open-cell frothed foams.
Abovementioned US 2005/100754 A indicates that “some” of its dispersions are “particularly suited for the formation of textile coatings including fabric impregnation” but does not indicate which ones, out of its great range of possible dispersions, should be particularly suited for such purpose.
The present invention seeks an improved conveyor belt suitable for tobacco and food applications.